fix screenreader bug for disabled submission button

closes LA-942
flag = none

Test Plan:
- Enabled Assignment Allowed attempts feature
- create an assignment with limited attempts
- as a student, submit the assignment
- attempt to submit again and the submission button should be disabled
- ScreenReader should now read the button as "dimmed" and should not
  be able to be clicked via screenreader shortcut

Change-Id: I05e2b3816de5bf63aa7aa1b915e8076e5efc4438
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/235237
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Ed Schiebel <eschiebel@instructure.com>
QA-Review: Daniel Sasaki <dsasaki@instructure.com>
Product-Review: Clint Furse <cfurse@instructure.com>

app/helpers/assignments_helper.rb

@@ -69,13 +69,12 @@ module AssignmentsHelper
     if assignment.expects_submission? && can_do(assignment, user, :submit)
       submit_text = user_submission.try(:has_submission?) ? I18n.t("Re-submit Assignment") : I18n.t("Submit Assignment")
       late = user_submission.try(:late?) ? "late" : ""
-      link_to(
-        submit_text,
-        '#',
-        :role => "button",
-        :class => "Button Button--primary submit_assignment_link #{late}",
-        :disabled => user_submission && user_submission.attempts_left == 0,
-      )
+      options = {
+        type: 'button',
+        class: "Button Button--primary submit_assignment_link #{late}",
+        disabled: user_submission && user_submission.attempts_left == 0
+      }
+      content_tag('button', submit_text, options)
     end
   end